1. Field of the Invention
The present invention relates to a waveguide as a transmission medium of an electromagnetic wave and, more particularly, to a waveguide for a millimeter wave frequency band or higher, which has a mechanical strength high enough to be mounted in a satellite.
2. Description of the Prior Art
Devices mounted in a satellite must have a mechanical strength high enough to withstand vibrations upon launching of a rocket, a shock upon separation of a booster, and the like, unlike those used on the earth. In general, such devices are required to have a mechanical strength high enough to withstand a random wave vibration level of 19.6 grms. For this reason, even a waveguide as a transmission medium of a radio wave adopts a flexible waveguide with flexibility, as disclosed in Japanese Utility Model Laid Open No. 57-62403.
FIGS. 1 and 2 are respectively a perspective view and a sectional view of a conventional flexible waveguide assembly.
As shown in FIGS. 1 and 2, a flexible waveguide assembly comprises a flexible waveguide 101, and rectangular waveguides 102 and 103, and waveguide flanges 104 and 105, which are connected to the two ends of the flexible waveguide 101. This prior art exemplifies a waveguide used in a frequency band from 26.5 to 40 GHz, and the dimensions of the waveguide are a long side a=7.1 mm, and a short side b=3.55 mm.
The aperture size of the waveguide becomes smaller as the transmission frequency becomes higher. For example, in the frequency band from 26.5 to 40 GHz like in the prior art, the dimensions are 7.1 mm.times.3.55 mm. When the frequency band is equal to or higher than 40 GHz, the long side x short side become as small as 5.7 mm .times.2.85 mm or less.
However, as described above, the flexible waveguide mounted in a satellite must have a mechanical strength high enough to withstand a random wave vibration level of 19.6 grms.
Therefore, when a flexible waveguide to be used in a millimeter wave frequency band exceeding 40 GHz is to be mounted in a satellite, a flexible waveguide which has a small aperture size and a sufficient mechanical strength must be realized, or a waveguide which has a sufficient mechanical strength and large dimensions must be used for the millimeter wave frequency band without any modifications.
However, it is very difficult to manufacture a flexible waveguide which has a small aperture size corresponding to the millimeter wave frequency band, and has a mechanical strength high enough to withstand a use in a satellite, and such a waveguide has not been realized yet.
When the conventional flexible waveguide assembly is used in the millimeter wave frequency band equal to or higher than 40 GHz, a transmission loss of a radio wave at a frequency of 43 GHz or higher and a transmission loss deviation in a frequency bandwidth of 200 MHz become too large for practical applications, as shown in FIG. 3. The measurement results were a transmission loss of 1.5 dB, and a transmission loss deviation of 1.3 dB in the frequency bandwidth of 200 MHz.
The performance generally required for a flexible waveguide assembly includes a transmission loss of 0.5 dB or less and a transmission loss deviation of 0.2 dB or less in the frequency bandwidth of 200 MHz. Thus, the performance of the conventional flexible waveguide assembly in the millimeter wave frequency band is considerably inferior to the required performance, and cannot be put into a practical application.